Amplitude modulator



May 1o, 1949.

E. R. SHENK AMPLITUDE MoDULAToR Filed July s, 1947 INVENTOR.

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ATTORN EY Patented May io, 1949 AMPLITUDE MODULATOB Eugene R. Shenk, Brooklyn, N. Y., assignor tov Radio Corporation of America, a corporation of Delaware Application Joly 9, i947, sel-iai No. 159,891

- Claims. 1

'I'his application discloses an improved carrier wave amplitude modulator linear over a wide amplitude modulation range. v

When the multi-grid type of mixer tube, for example, the type 6SA7, is used as an amplitude modulator, the resulting circuit has two major disadvantages.

a. The percent of linearvamplitude modulationv that can be realized is limited to approximately b. For given circuit conditions, the percent of amplitude modulation obtained varies widely from tube to tube, although the tubes check satisfactory on normal mutual conductance tests.

(Variations'from 28% to 70% have been meas-v ured.) In some applications, this requires that a percent modulation adjustment mechanism be included which can be reset each time the tube is replaced. l

An object of my invention is to provide an amplitude modulation circuit wherein linear modulation percentages of 90% or more can be obtained. It is a further purpose to provide an amplitude' modulator circuit in which replacement of tubes may be designated the( modulator tube, Rec.

tangular waves of carrier wave frequency and negative polarity only are applied to the grid of the other tube. Themodulating signal varies the instantaneous bias in the grid circuit of this other tube to modulate the carrier output amplitude. The output of a band pass filter in the plate circuit of the carrier amplifier providesthe amplitude modulated signal.v

In describing my invention in detail, reference will be made to the attached drawings wherein,

Fig. 1 illustrates by circuit element and circuit element connection, an improved modulator arranged in accordance with my invention,

Figs. 2 illustrate by voltage curves, the operation of the modulator of Fig. 1, and

Figs. 3 and 4 illustrate by wave form the oper- 2 ation of the improved modulator of Fig. 1, when the signal input at Il is oi sine wave form.

In Fig. 1, lil is a source o! wave energy of rectangular wave form and carrier wave frequency such as supplied by an amplitude limiting vacuum tube excited by oscillations of' said carrier irequency. The source lll is connected to the cathode I2 of a diode DI, the anode I3 of which'is coupled to the control grid i4 ot a tube V2. The

D. C. return for grid il is supplied by resistor R6. The cathode IS of the tube V2 is connected to the cathode I6 of modulator tube Vl by resistors R2 and R3: The point intermediate these resistors is connected to ground by resistor R4. Modulationis supplied from a source I8 to the control g-rid 20 of tube VI. The D. C. return for the control grid 20 of tube Vi is by resistor RI. Anode direct current potential from a' source +B is supplied directly to the anode 22 of tube Vi and by resistor R5 to the anode 28 of tube V2. Output maybe taken from the anode 28 and if desired, a iilter circuit 30 may be included in the output.

Carrier frequency input is applied from source i0 to the grid il of tube V2 through a rectifier `which, for'convenience only, is shown as a diode tube and herein is referred to as DI. 'I'his diode `is so poled as to pass only negative excursions of the carrier voltage to thus prevent the carrier frequency voltage from driving the grid of 'V2 in a positive direction.

The instantaneous cathode l5 voltage of tube V2 is dependent -on the plate current of tube Vi. This plate' current is, in turn, under control of the modulating signal input voltage applied to the grid 20 of tube Vi. Itis apparent, therefore, that the cathode voltage of tube V2 is ylargely controlled by the modulating signal input to the grid of tube Vi.

Assume the signal voltage applied to the grid 20 of tube Vi is zero.' Assume further that a large amplitude of carrier frequency voltage is applied at the point indicated, the amplitude being sumcient to operate the grid of tube V2 to well beyond the plate current cut oil value on succes- `sive negative half cycles. Under these conditions, the plate voltage wave form of tube V2 will appear as in Fig. 2a.

VApplication ofa negative voltage-to the grid 2D of tube Vl will reduce thecathode i5 voltage of tubeVZ. This is equivalent to reducing the negative bias on the grid il of tube V2. Consequently, V2'will pass a higher value of plate current during its conductive periods and its plate they isolateeach tube from the other tube.

' TVI, then the outputwaveform at the plate of tube V2 will be as in Fig. 3 (assuming, of course, that the Vcarrier frequency is higher than the modulating frequency). By applying this voltage Wave to the input of a conventional band pass filter of proper characteristics, the wave form of Fig. 4 is obtained at the output of the filter.

It will be clear from the simple examples given above that any voltage wave form applied to the grid 2li of tube Vi will be reproduced as the envelope of the amplitude modulated wave at the output terminals of the band pass lter.

Resistors R2 and R3 are not essential to the operation of the modulator. However, their presence is considered very desirable, because Each triode tube sees the other tube as a part of its cathode load impedance. The fraction of this ioad that is represented by the tube is smaller when R2 and R3 are present. Consequently, a change in the characteristics of either tube has less net eii'ect on the operation of the circuit.

` Itis also well known that a resistor in the cathode circuit of a tube stabilizes the characteristics of the tube itself by virtue-of a phenomenon known as current feedback.

What is claimed is:

i. In a linear high percentage amplitude modulation system, a carrier amplifier tube, a modulator tube, a direct current potential source for both tubes including a common resistor between said source and both cathodes and a second resistor between the said source and the ,anode of the first tube, the arrangement being such as to' cause potential variations on one cathode to appear on the other cathode, a source of oscillatory energy of carrier wave frequency coupled to the control grid of the first tube, a source of potentials representing signals coupled to the control grid of the second tube, and an output circuit coupled to said second resistor.

2. A system as recited in claim 1 including a half wave rectier inthe coupling between said source of energy and the'control grid of the first tube.

3. In a high percentage carrier amplitude modulation system, a carrier amplifier tube having an anode, a control grid and a cathode, a modulator tube having an anode, a cathode and a control grid, a common resistor connected between the cathodes of both of said tubes and the control grids of both of said tubes to cause potential variations on` one cathode to appear anode and `cathode of both tubes, a source of oscillatory energy of carrier wave frequency coupled to the control grid of vthe first tube, a half wave rectifier in said last named coupling so poled as to permit half cycles only of said carrier wave energy to reach said control grid, a source of potentials representing signals coupled to the control grid ofthe second tube, an output circuit coupled to the anode of the first tube, and a filter in said last named coupling.

4. In a. high percentage linear amplitude modulation system, a carrier amplifier tube having electrodes including an anode, a control grid and a cathode, a modulator tube having electrodes including an anode, a cathode and a control grid, a direct current circuit for said tubes including a source of direct current potential connected in series with the anode to cathode impedances of said tubes, a connection between the cathodes of said tubes and a resistor in a common cathode return so that when'the potential on one cathode changes a corresponding change takes place in the potential on the other Y cathode, a source of oscillatory energy of carrier wave frequency coupled to the control grid of the first tube, a unidirectional valve in said last named coupling so that half cycles only of the carrier energy from said source reach said control grid, a source of potentials representing signals coupled to the control grid of the second or modulator tube, and an output circuit coupled to the anode of the first tube.

5. In a linear high percentage modulation system, a carrier amplifier tube having an anode, a control grid and a cathode, a. modulator tube having an anode, a cathode and a control grid, a coupling between the cathodes of said tubes and their grids including two resistors in series between the cathodes1 a common resistor conf necting the junction point of said two resistors -to the grids of both tubes to cause potential No references cited. 

